You could take it from either of the non-right angles.
To one angle, which side is opposite and which is adjacent is different than to the other angle. This is also why sin(x) = cos(90-x) and cos(x) = sin(90-x).
4y - 3 = y + 6
Add 3 to both sides
4y = y + 9
Subtract y from both sides
3y = 9
Divide both sides by 3
y= 3
Answer: 1/2
Step-by-step explanation: y = (1/2)x has the format y = kx, where k is the const. of prop. In this particular case that const. of prop. is (1/2).